1. Field of the Invention
The present invention relates to novolac polymer planarization films for microelectronic structures, such as integrated circuits. More particularly, this invention relates to an improvement in the formation of a planarization film which remains adhered to substrates upon exposure to heat.
2. Background of the Invention
Novolac polymers have been used extensively in the manufacture of substrates, such as multichip modules, printed circuit boards, integrated circuits and other microelectronic devices. For example, photoresists used for microlithographic patterning of circuitry often contain a novolac component. See e.g., Wolf, et al., "Silicon Processing for the VLSI Era, 1 Process Technology" 418 (1986).
In addition, novolac polymers are also contained within planarizing films used in the fabrication of microelectronic devices. As used herein, the term "planarizing film" refers to a film coated onto a patterned substrate surface for the purpose of smoothing the topography of the substrate.
The planarization or smoothing of surfaces is essential in the fabrication of integrated circuits. For example, as optical lithography is used to define even smaller features, the depth of focus of the exposure tool will decrease. Therefore, it is necessary to employ planarizing films to smooth or "level" the topography of microelectronic devices in order to properly pattern the increasingly more complex integrated circuits. See e.g., Stillwagon, et al., Evaluation of Several Organic Materials as Planarizing Layers for Lithographic and Etchback Processing, 412 Polymers in Micrography, ACS Symp. Series, 252-265 (1989).
For a film to be useful for planarization, it must continuously and completely cover a substrate, such as a silicon wafer, when the latter is subjected to temperatures ranging between about 20.degree. C. and about 300.degree. C., to vacuum pressures, and to etching processes, such as wet chemical or plasma. As used herein, "continuously" means that the film does not have "bare spots" whereat the substrate surface is revealed.
Low number average molecular weight novolac polymers, i.e. those ranging between about 200 and about 2000 atomic mass units ("amu"), are especially useful in forming planarizing films because they tend to flow more readily than other polymers having higher molecular weights. Unless indicated otherwise, all references herein to "molecular weight" are to be construed as number average molecular weight. Such polymers having low viscosities, i.e. between about 10 centipoises and about 2000 centipoises at temperatures in the range of about 120.degree. C. to about 150.degree. C., are preferred since, when dried, they form either locally, regionally or globally planarized films on patterned substrates.
As used herein, the term "local planarization" refers to a condition wherein the film is planar or flat over a distance of 0 to about 5 linear micrometers. "Regional planarization" refers to a condition wherein the film is planar or flat over a distance of at least about 5 to about 50 linear micrometers on the substrate surface. "Global planarization" refers to the same situation over larger substrate surfaces, i.e. about 50 to about 1000 linear micrometers.
After the polymer material is applied to the substrate by conventional spinning techniques, the polymer-substrate composition is heated to evaporate any residual solvent present within the film material, to further reduce the viscosity of the film, and to enhance leveling of the film on the substrate. One difficulty in achieving the desired planarization is caused by the delamination of film from the substrate during this heating process. Another difficulty is that the evaporation of the solvent from the polymer solution, which occurs after the solution is spun across the substrate surface, causes a reduction in the total volume of the planarization material which, in turn, results in non-uniform coatings.
One method that has been suggested for improving the adhesion of films to substrates is to employ substrate surface modification, whereby the substrate is coated with a thin layer of an adhesion promoter, such as 3-aminopropyltriethoxysilane. However, this technique is undesirable because it adds extra steps and costs into the film formation process.
Other commonly used techniques, such as chemical and plasma surface cleaning treatments or surface roughening treatments, i.e. ion bombardment, have been used for the purpose of improving the adhesion of the film to the substrate by way of increasing the contact area therebetween. However, these techniques are not only more costly and time consuming, but also the substrate often incurs radiation damage from the plasma source.
The use of surfactants in novolac polymer solutions especially formulated for film formation is known. However, these surfactant-containing solutions are comprised of novolac polymers having higher molecular weights, i.e. novolacs which are greater than 10,000 amu, and are only useful in the formation of photoresists, as opposed to the formation of planarizing films. These solutions contain only small amounts of surfactants primarily for the purpose of preventing striations or irregularities in the novolac or photoresist film thicknesses.
It would be desirable to provide an improved process for forming continuous, uniform planarization films whereby the films produced therefrom would be not only useful for achieving regional or global planarization of microelectronic substrates, but also would not delaminate from the substrates upon exposure to heat. It would also be desirable to provide a composition which can be used to form such planarized films.